A few suggestions:
Afternoon shows like Mac King
Mob Museum downtown
Exhibit at the Venetian--They currently have Leonardo Da Vinci exhibit that is very interesting
Atomic Testing Museum--east of the strip on Flamingo
Pin Ball Hall of Fame
Shoot some automatic weapons at one of the several gun ranges in town.
Visit the Art Gallery at the Bellagio
Take in the seasonal display at the Bellagio conservatory
Visit the Antique car museum on the 5th floor of the Linq (formerly Quad, formerly Imperial Palace)
Arrange a visit to a Spa
Pool time when the weather permits
Shark Reef at Mandalay Bay
Bodies or Titanic exhibits at the Luxor
Take a nap
Rent a car and drive out to: Hoover Dam, Red Rock Canyon, Mt Charleston, Valley of Fire State Park,
or visit the Winery in Pahrump.
Take a trip up to the top of the Strat or Eifel Tower at the Paris casino
Find yourself a casino bar, have a few brews and people watch--my favorite afternoon watering hole is
the bar at Margaritaville or Hogs and Heifers if I am downtown.
Visit and get a picture of yourself in front of the "Welcome to Las Vegas" sign
Ride the roller coaster at NYNY
Hey, it's Vegas there is always something to do and see in this town!